Method and apparatus for quantitatively evaluating the stator wedge tightness of an electric alternator

Abstract

Disclosed is a method for quantitatively evaluating the tightness of the stator wedges of an alternator, which can be reduced to practice without having to remove or dismantle the rotor of the alternator even when the air-gap of the alternator is as small as 10 mm, and which permits to obtain a quantitative evaluation of the compression percentage of the ripple spring holding the wedges in a simple, efficient and reliable manner. This method makes use of a thin flat sensor having one face provided with a recess in which a piston is mounted. This sensor is inserted into the air-gap of the alternator and positioned in front of the stator wedge to be evaluated so that the piston faces this wedge. Thus, a fluid is injected into the sensor to bring the piston to contact and then press against the wedge while the sensor is backed against the rotor. The pressure of the injected fluid which is proportional to the force applied to the ripple spring, is measured together with the displacement of the stator wedge while the pressure is exerted and the so-measured force and displacement are used to determine the tightness of the wedge. The above mentioned measurement of the displacement of the wedge is achieved by measuring the displacement of the piston in its cylinder by means of strain gauge fixed onto resilient blades bearing against the piston. An apparatus for carrying out this method is also disclosed.

We claim: 
     
       1. A method for quantitatively evaluating the stator wedge tightness of an alternator comprising a rotor and a stator together defining an air-gap of a given width, said stator being provided with radial slots having outlets in communication with the air-gap and in each of which is retained at least one stator coil by means of a resilient holding means which abuts against stator wedges slidably inserted into a set of symmetrical grooves provided therefor in the stator at the outlet of each slot, said method comprising the steps of: (a) inserting into the air-gap a sensor provided with a mobile element capable of being displaced in a transverse direction relative to said air-gap by the effect of a pressure exerted by a fluid;   (b) positioning the sensor in front of the stator wedge the tightness of which is to be evaluated in such a manner that said mobile element faces said stator wedge;   (c) exerting pressure on said fluid to displace the mobile element of the sensor and to cause this element to contact first and then press against said stator wedge while said sensor is backed against said rotor;   (d) measuring the so-exerted pressure, said pressure being proportional to the force applied to said resilient holding means behind the stator wedge;   (e) measuring the displacement of the stator wedge as the pressure is exerted; and   (f) determining the tightness of the stator wedge from the so measured force and displacement; wherein:     (g) the sensor that is used, has a body in the shape of a thin plate having a thickness slightly smaller than the width of said air-gap, said plate having one face provided with a recess acting as a cylinder in which is mounted a piston acting as said mobile element, said piston being remotely actuatable by injection of said pressurized fluid into said cylinder; and   h) the measurement of the displacement of the wedge is achieved by measuring the displacement of the piston in said cylinder by means of at least one resilient blade having a first extremity rigidly fixed to the body of the device; a second extremity mounted on the piston so as to be displaced with said piston, and a central portion on which is attached at least one strain gauge giving a signal proportional to the displacement of said piston and thence to the displacement of said stator wedge when the latter is displaced.   
     
     
       2. The method of claim 1, wherein, before carrying out steps (b) to (f), said sensor is positioned in the air-gap between rigid surfaces of the rotor and the stator and the value of the displacement of the piston corresponding to the working pressures is measured, and wherein, in step (f), the so measured displacement value is subtracted from the measured value of the same displacement when pressure is exerted on the stator wedge. 
     
     
       3. The method of claim 1, wherein, when step (b) is completed, any variation in the width of the air gap is measured by at least one strain gauge mounted on a resilient blade having one end fixed to the body of the sensor and another free end provided with a shoe that contacts and slides onto a rigid surface of the stator, and wherein, in step (f), the so-measured variation is substracted from the measured value of the displacement in order to obtain the actual value of the displacement of the stator wedge when pressure is exerted onto said stator wedge. 
     
     
       4. The method of claim 1, wherein step (b) is carried out by mechanical means devised to displace said sensor along each radial slot. 
     
     
       5. The method according to claim 4, wherein said mechanical means are controlled to automatically displace the sensor along each radial slot so that said sensor is positioned in front of each successive stator wedge slidably inserted into the grooves made in said radial slot. 
     
     
       6. The method of claim 5, wherein said mechanical means are also devised to displace the sensor from one given slot to at least one other adjacent slot. 
     
     
       7. The method of claim 4, wherein said mechanical means are manually controlled and wherein said sensor is provided with a optic fiber visualization system allowing an operator to locate each stator wedge and to position the sensor relative to said stator wedge. 
     
     
       8. The method of claim 1, wherein the fluid used is compressed air. 
     
     
       9. The method of claim 1, wherein step (f) consists in determining the percentage of compression of the resilient holding means by exerting a constant pressure during step (c) and by measuring the displacement that is so-obtained. 
     
     
       10. The method of claim 1, wherein step (f) consists in determining the percentage of compression of the resilient holding means by setting a given displacement and by measuring the pressure that is required to achieve such a displacement. 
     
     
       11. The method of claim 1, wherein step (f) consists in determining the percentage of compression of the resilient holding means by setting a threshold value corresponding to an ordinate of the curves giving the force as a function of the displacement, such an ordinate corresponding to a requested equivalent percentage of compression, and then by verifying if the measured value of the force that is exerted has reached or not this threshold value. 
     
     
       12. An apparatus for quantitatively evaluating the stator wedge tightness of an alternator comprising a rotor and a stator together defining an air-gap of a given width, said stator being provided with radial slots having outlets in open communication with said air-gap and in each of which is retained at least one stator coil by means of resilient holding means abutting stator wedges slidably inserted into a set of symmetric grooves provided therefor in the stator at the outlet of each slot, said apparatus comprising: (a) a sensor dimensioned so as to be insertable into said air-gap, said sensor being provided with a mobile element capable of displacement in a direction transverse to said air-gap by the effect of a pressure exerted by a fluid;   (b) means for positioning said sensor in front of a stator wedge where the tightness evaluation is required, thereby allowing said mobile element of said sensor to face said stator wedge;   (c) means for exerting pressure by means of a fluid to displace the mobile element of said sensor and cause it to press against said stator wedge while said sensor is backed against the rotor;   (d) means for measuring the pressure that is exerted when the mobile element presses against said stator wedge, such pressure being proportional to the force applied to said resilient holding means behind said stator wedge;   (e) means for measuring the displacement of the stator wedge as the pressure is being exerted; and   (f) means for evaluating the tightness of the stator wedge by means of the so-measured force and displacement; wherein:     (g) said sensor has a body in the shape of a thin plate having a thickness slightly smaller than the width of said air-gap, said body having one face provided with a recess acting as a cylinder in which is mounted a piston acting as said mobile element, said piston being remotely controlled by injection of said fluid under pressure into said cylinder; and   (h) said means for measuring the displacement of a stator wedge consists of means for measuring the displacement of the piston in said cylinder, said piston deplacement measuring means consisting of at least one resilient blade having a first extremity rigidly fixed to the body of the sensor, a second extremity mounted on the piston so as to be displaced therewith, and a central portion on which is attached at least one strain gauge giving a signal proportional to the displacement of said piston and thence to the displacement of the stator wedge when the latter is displaced.   
     
     
       13. The apparatus of claim 12, wherein said means (b) comprises a first mechanical system allowing displacement of said sensor along each radial slot. 
     
     
       14. The apparatus of claim 13, wherein said first mechanical system is operated by a motor which is controlled by a control device for automatically displacing said sensor along each radial slot, whereby the sensor can be positioned in front of each successive stator wedge slidably inserted in the grooves of said radial slot. 
     
     
       15. The apparatus of claim 14, wherein the first mechanical system includes a chain rolled on a drum driven by the motor, said chain having a free end unrollable into the air-gap and to which the sensor is attached. 
     
     
       16. The apparatus of claim 14, wherein said means (b) also comprises a second mechanical system allowing displacement of the sensor from a given slot to at least one other adjacent slot. 
     
     
       17. The apparatus of claim 16, wherein said second mechanical system includes a carriage on which the first mechanical system is mounted and means to displace the carriage in translation. 
     
     
       18. The apparatus of claim 14, wherein the first mechanical system is manually controlled and the sensor is provided with a fiber visualization system allowing an operator to locate each stator wedge and to position the sensor relative to the same. 
     
     
       19. The apparatus of claim 18, wherein said first mechanical system includes a chain rolled on a drum driven by a motor, the chain having a free end unrollable into said air-gap and to which is fixed the sensor. 
     
     
       20. The apparatus of claim 18, wherein visualization system comprises an optic fiber having an extremity adapted to serve as a spectacle and which is fixed to said sensor. 
     
     
       21. The apparatus of claim 20, wherein the extremity of the optic fiber serving as a spectacle extends transversely to the piston in a cavity provided in the body of the sensor, and faces a mirror incorporated in said cavity, and wherein lighting means are provided at the level of the said sensor to facilitate the visualization of a stator wedge the tightness of which is to be evaluated, with said optic fiber. 
     
     
       22. The apparatus of claim 12, wherein the apparatus further comprises at least one spacing plate attachable to the body of said sensor opposite to the face where the piston is mounted, whereby the thickness of the sensor is increased, allowing use of the sensor in an air-gap of larger width. 
     
     
       23. The apparatus of claim 12, wherein said means (f) includes an electronic conditioning system and a computer processing system. 
     
     
       24. The apparatus of claim 23, wherein said injected fluid is compressed air and the apparatus is in the form of a portable kit.